Polysaccharides of General formula Y
Polysaccharides have a wide range of functions in organisms, such as building, stocking, or regulatory function. The polysaccharides of the general formula Y also belong to the polymers naturally occurring in organisms.

where R represents CH3—CO—NH— or —OH, and R1 represents —SO2—ONa, —SO2—OH, or —H. This comprises for example chondroitin sulfate, dermatan sulfate, carrageenan, keratan sulfate, or hyaluronic acid.
Chondroitin sulfate is linear, sulfated, and negatively charged glycosaminoglycan composed of repeating monomer units of N-acetyl-D-galactosamine and D-glucuronic acid, linked with each other by β(1→3) and β(1→4) O-glycosidic bonds (the structural formula of chondroitin sulphate see below),

where
R1 is —H or —Na,
R2 is —H, —SO2—ONa or —SO2—OH.
Chondroitin sulfate is derived from animal connective tissues where it is bound on proteins and thus forms part of proteoglycans. The sulfation of chondroitin is realized by means of sulfotransferases in various positions and by various types. The unique pattern of sulfation in particular positions in the polymer chain encodes the specific biologic activity of chondroitin sulfate. Chondroitin sulphate is an important structural block of cartilage in joints, providing them with compressive strength and renewing the balance of joint lubrication fluid composition (Baeurle S. A. a kol. Polymer 50, 1805, 2009).
Dermatan sulfate is linear, sulfated, and negatively charged glycosaminoglycan composed of repeating monomer units of N-acetyl-D-galactosamine and L-iduronic acid, linked with each other by β(1→3) and β(1→4) O-glycosidic bonds (the structural formula of dermatan sulfate see below),

where
R1 is —H or —Na,
R2 is —H, —SO2—OH or —SO2—ONa.
Dermatan sulfate differs from chondroitin sulfate by the presence of L-iduronic acid that is C-5 epimere of D-glukuronic acid. The inverse configuration of iduronic acid confers better flexibility to dermatan sulfate chains and ensures their specific glycosaminoglycan-protein interaction in the surrounding area. These interactions contribute to the regulation of several cell processes, such as migration, proliferation, differentiation, or angiogenesis. The conversion of chondroitin sulfate into dermatan sulfate is ensured by means of three enzymes, namely dermatan sulfate epimerase 1 (DS-epi1), dermatan sulfate epimerase 2 (DS-epi2), and dermatan 4-O-sulfotransferase (D4ST1) (Thelin M., et al. FEBS Journal 280, 2431, 2013).
Keratan sulfate belongs to the group of linear sulfated polysaccharides comprising D-galactose, N-acetylglucosamine, and galactose-6-sulfate linked with β(1→3) and β(1→4) bonds, having the structure and bonds similar to chondroitin sulfate. It can be found in cornea, cartilages, bones, and connective tissue (the structural formula of keratan sulfate see below),

where R2 is —H, —SO2—OH, or —SO2—ONa.
Carrageenans belong to the group of linear sulfated polysaccharides that are obtained by the extraction of red sea algae. Galactose and its 3,6-anhydro derivative are their basic structural units, linked to each other by β(1→3) and β(1→4) O-glycosidic bonds. There exist three main groups of carrageenans that differ in the degree of sulfation and water solubility. Kappa-carrageenan has one sulfate group in dimer and forms stiff gels in aqueous environment. Iota-carrageenan contains two sulfates and forms soft gels, whereas lambda-carrageenan having three sulfates does not exhibit any gelling properties.
Hylauronic acid is non-sulfated glycosaminoglycan composed of two repeating units of D-glucuronic acid and N-acetyl-D-glucosamine.

where
R1 is H or Na.
The molecular weight of native hyaluronic acid is in the range of 5.104 to 5.106 g·mol−1. This very hydrophilic polysaccharide is a part of connective tissues, skin, joint synovial fluid; it plays an important role in many biological processes, such as proteoglycanes organisation, cells hydration and differentiation. As this polymer occurs naturally in the body and thus it is biodegradable, it is useful as a substrate in the field of tissue engineering or as a carrier of biologically active substances.
Polysaccharides Containing Multiple Bonds
Polysaccharides having —C═C— multiple bond that forms a part of saccharide cycle and is not situated at the end of the chain are very rare. As an example of common polysaccharides, 5,6-unsaturated derivatives of cellulose, known as cellulosenes (Vigo T. L. et al. Polymers for Advanced Technologies, 10, 6, 311-320, 1999) or their 2,3-unsaturated analogues are described. The method of the preparation of 5,6-unsaturated derivatives is based on elimination reaction of the leaving group in the position 6 and hydrogen in the position 5 under basic conditions to produce enol ether (—C═C— multiple bond is conjugated with heterocyclic oxygen). The preparation of 2,3-unsaturated derivatives of cellulose, amylose, or xylan (D. Horton et al. Carbohydrate Research, 40, 2, 345-352, 1975) requires, in addition to the presence of leaving groups, a reducing agent, usually zinc, to produce a standard alkene (without the conjugation of multiple bond and oxygen).
Utilization of polysaccharides Containing Multiple Bonds
The applications are aimed mostly at the modification of —C═C— multiple bond that is not a direct part of the saccharide cycle. These methods are based on bonding a new substance to the polymer skeleton, where the structure is changed in an important way, i.e. the native character of the polysaccharide is lost. In such cases, the multiple bond is normally used in polymerization (Bellini D. WO96/37519), addition (Khetan S. et al. Soft Matter, 5, 1601-1606, 2009) or cycloaddition reactions (Nimmo Ch. M. et al. Biomacromolecules, 12, 824-830, 2011; Bobula, T. et al. Carbohydrate. Polymers, 125, 153-160, 2015). These methods can lead to both effective crosslinking of polysaccharides (Collins M. N. et al. Carbohydrate Polymers, 92, 1262-1279, 2013; Hacker M. C. et al., Inter. J, of Mol. Sc., 16, 27677-706, 2015), and selective bonding of active substances to the polymer (Mero A. et al. Polymers, 6, 346-369, 2014). The multiple bond of methacrylate group is also very commonly used to perform a polymerization reaction (Granstrom M. a kol. EP2899214).
There are only a few methods of introducing the multiple —C═C— bond directly into the saccharide cycle in a polymer chain. One of them is an enzymatic cleavage of polymers by lyases, where the double bond is formed at the non-reducing end of the polymer (Kelly S. J. a kol. Glycobiology, 11, 4, 294-304, 2001), which is, in this case, hyaluronic acid (see the scheme below).

It means that the occurrence of this modification strongly depends on the molecular weight; for example, for the molecular weight of 4.104 g·mol−1 only one of one hundred disaccharides is modified, if the molecular weight is 4.105 g·mol−1, one of one thousand disaccharides is modified. Thus, it is obvious that, except for polysaccharide oligomers, this type of modification is minor and there is actually no difference between the starting and the resulting high molecular polymer.
The second method enables incorporating the double bond into the polysaccharide structure in the positions 4 and 5 along the whole chain length, so the polymers having a higher molecular weight can be efficiently modified (Buffa R. et al. WO2014/023272, Bobula T. et al. Carbohydrate Polymers, 136, 1002-1009, 2016). However, in this method, multiple —C═C— bond is formed, and this bond is directly conjugated with strong electron acceptor aldehyde group. This modification significantly changes chemical properties of the polysaccharide, because it enables covalent bonding of wide range of nucleophiles, usually amines. The above mentioned fact also implies that the polymer modified in this way has significantly higher electrophile characteristic and thus it chemically differs from the native polymer. It can be also considered as less active antioxidant comparing to the non-modified polymer.
The solution described in this invention can eliminate these drawbacks; there is no reactive electrophile group in the modified polymer, compared to the non-modified polymer. On the contrary, the double bond conjugated with heterocyclic oxygen can be considered as a group having nucleophilic (antioxidant) properties.